The isomorphous title compounds (and the ordered substitutional Rb14CsTl27) are obtained directly from reactions of the elements in sealed Ta below similar to 330 degrees C. Refinements of single-crystal data for the three established a structure with alternate layers of isolated pentacapped trigonal prismatic Tl-11(7-) (D-3h) ions and condensed (2.)(infinity)[Tl-16(8-)] networks that are separated by cations. The condensed layer consists of Tl-11 units that share prismatic edges and ate interbridged through waist-capping atoms (Tl6/2Tl3Tl2). (Rb15Tl27 : P (6) over bar 2m, Z = 1, a = 10.3248(6) Angstrom, c = 17.558(2) Angstrom.) The rubidium phase is a poor metal (p(293) similar to 34 mu Omega.cm) and is Pauli-paramagnetic. Extended Huckel band calculations indicate partially filled bands and a non-zero DOS at E(F), consistent with the observed metallic behavior, although appropriate cation tuning or modest anion doping should provide a Zintl phase. The band structure and COOP curves are also used to rationalize the distortion of the Tl-11 unit on condensation and the critical role of the interfragment bonds between waist-capping atoms in stabilizing the layer.